Jet fuel is much like diesel fuel and is classified as a low temperature burning fuel.
The Buildings did fall straight down, bearing a classic crimp just before implosion, with squibs visible as implosion occured, falling vertically onto its own footprint, almost at the rate of free fall, just like any well planned controlled demolition.
Fires were visibly minimal at the time of collapse.
http://www.physics.byu.edu/research/energy/htm7.html